The present invention relates to a communication system comprising a first node having compression means for deriving from a plurality of source signals a plurality of compressed source signals and a multiplexer for multiplexing said plurality of compressed source signals into a multiplex signal, the first node comprises transmission means for transmitting the multiplex signal to a second node, the second node comprises a demultiplexer for demultiplexing the multiplex signal into said plurality of compressed source signals.
The present invention also relates to a node for use in such a communication system.
A communication system according to the preamble is known from International Patent Application published under No. WO 98/16083.
In the past networked PBX""s have been introduced, in which a plurality of PBX""s are interconnected via a communication link in order to obtain a larger virtual switch. The PBX""s can be at different locations, which can be in different cities and even in different countries. The communication link often comprises a connection over a public telephone network (PSTN).
A substantial part of the operational costs of the network of interconnected PBX""s is constituted by the costs of the interconnections between the PBX""s charged by the operator of the public telephone network. To reduce said costs often compression means are used to compress the signals to be transmitted, making that more signals can be transmitted over a link having a predetermined capacity. In case of voice signals the speech signal is dependent on the compression ratio used in the compression means. The use of compression will result in a degradation of the speech quality which depends on the amount of compression. Consequently the use of compression will result in some degradation of the transmission quality of the communication system.
The object of the present invention is to provide a communication system according to the preamble in which the transmission quality of the communication system has been improved.
To achieve said object, the communication system is characterized in that the compression means are arranged as variable compression means for compressing the source signals in dependence on an aggregate rate of the source signals to be transmitted and in that the decompression means are arranged as variable decompression means for decompressing the signals received from the demultiplexer to obtain the source signals.
If the compression is made dependent on an aggregate rate of the signals to be transmitted over a given communication link, it is obtained that the compression ratio is never more that is really needed to be able to transmit the required amount of signals over the transmission link. If there is heavy traffic a large compression ratio is used, resulting in some degradation of the quality of the transmitted signals. If there is only light traffic, a small compression ratio or even no compression at all is used, resulting in an improvement of the quality of the transmitted signals.
An embodiment of the invention is characterized in that the compression means are arranged for selecting one out of a plurality of compression ratios for the source signals.
The adjustment of the compression ratio by selecting one out of a plurality of compression ratios can easily be done by changing the compression and decompression methods. This is easier to perform than gradually changing the compression ratio of a single compression method.
A further embodiment of the invention is characterized in that said plurality of compression ratios differ by an integer factor. By choosing compression ratios that differ by an integer factor, it becomes easier to add an additional source signal to the multiplex signal to be transmitted. It is then possible to replace one existing signal in the multiplex signal by two or more signals having the same aggregate bitrate than the single signal that is replaced. The two or more signals in the multiplex can now be used to carry the replaced source signal and the additional source signal.